Multi stage amplifier

ABSTRACT

A multi-stage amplifier includes a first, a second, and a third sub-amplifier, each with respective input and output ports. The multi-stage amplifier also includes a common output port. The output port of the second sub-amplifier is connected to the output port of the first sub-amplifier as well as to the common output port of the multi-stage amplifier, and the output port of the third sub-amplifier is connected to the common output port. The electrical lengths of the connections from the second sub-amplifier&#39;s output port both to the first amplifier&#39;s output port and to the common output port are longer or shorter than one quarter of a wavelength (λ) of the frequency for which the multi-stage amplifier is intended to operate.

TECHNICAL FIELD

The present invention discloses an improved multi-stage amplifier whichhas Chireix properties.

BACKGROUND

So called Chireix amplifiers are examples of RF amplifiers which arebased on multiple transistors with passive output network interactionwhich gives a high average efficiency for amplitude-modulated signals.

Such amplifiers, i.e. amplifiers based on passive output networkinteraction structures have a general advantage in that they need onlyfundamental (i.e. RF) frequency network and signal modifications;compared to single-transistor amplifiers they differ in the number ofindependently driven transistors. Many other types of high-efficiencyamplifiers require harmonics and/or baseband modifications, featureswhich are optional for amplifiers based on passive output networkinteraction structures.

However, one problem in amplifiers based on passive output networkinteraction structures is transistor shunt loss. This loss is due toresistive parasitics that couple the output node of the devices toground. This problem is emphasized by the operation of multi-transistoramplifiers, which decrease the transistors' RF output currents (which isthe reason for their high efficiency) at the expense of increased RFoutput voltages.

SUMMARY

As explained above, there is a need for an amplifier which is based onmultiple transistors with passive output network interaction which haslower shunt losses than previously known such transistors.

This need is addressed by the present invention in that it discloses amulti-stage amplifier which comprises a first, a second and a thirdsub-amplifier, each of which has an input port and an output port. Inaddition, the multi-stage amplifier of the invention also has a commonoutput port.

In the multi stage amplifier which is disclosed by the invention, theoutput port of the second sub-amplifier is connected to the output portof the first sub-amplifier as well as to the common output port of themulti-stage amplifier and the output port of the third sub-amplifier isconnected to the common output port.

The electrical length of the connections from the second sub-amplifier'soutput port both to the first amplifier's output port and to the commonoutput port are longer or shorter than one quarter of a wavelength ofthe frequency for which the multi-stage amplifier is intended tooperate.

As will be shown in the detailed description of this text, by means ofthis design of the amplifier of the invention, a high degree ofefficiency is obtained.

In one embodiment of the invention, the multi-stage amplifieradditionally comprises a fourth sub-amplifier with an input port and anoutput port which is connected to the output port of the thirdsub-amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, withreference to the appended drawings, in which

FIG. 1 shows one embodiment of the invention, and

FIGS. 2-5 show characteristics obtained by the embodiment of FIG. 1, and

FIG. 6 shows a second embodiment of the invention, and

FIGS. 7 and 8 show characteristic of the embodiment of FIG. 6, and

FIGS. 9 and 10 show possible circuit designs for use in the invention.

DETAILED DESCRIPTION

FIG. 1 shows a schematic view of a first embodiment 100 of a multi stageamplifier of the invention: as can be seen here, the multi stageamplifier 100 of the invention in this embodiment comprises a firstsub-amplifier 110, which has an input port 111 and an output port 112.

The multi stage amplifier 100 also comprises a second sub amplifier 120,which has an input port 121 and an output port 122. As can be seen inFIG. 1, the output port 112 of the first sub-amplifier 110 is connectedto the output port 122 of the second sub-amplifier 120 via a connection113 which has an electrical length L1.

In addition, the output port 122 of the second sub-amplifier isconnected to a common output port 140 for the entire multi-stageamplifier 100 via a connection 123 which has an electrical length L2.Also shown symbolically in FIG. 1 is an external load L, shown as 145,to which the multi stage amplifier 100 is connected.

The multi stage amplifier 100 of the invention also comprises a thirdsub-amplifier 130, with an input port 131 and an output port 132. Theoutput port 132 of the third sub-amplifier 130 is connected to thecommon output port 140 via a connection 133 with an electrical lengthL3.

In order for the desired lower losses to be obtained in the amplifier100, the first sub-amplifier 110 should generally be smaller than thesecond sub-amplifier 120 in terms of peak output power

The first sub-amplifier 110 may also be seen as an “outer” sub-amplifierin the circuit 100, while the second sub-amplifier 120 is seen as an“inner” sub-amplifier in the same circuit 100.

According to the present invention, the electrical lengths L1 and L2 arechosen so that their combination gives a useful load transformation forthe outer sub-amplifier 110, while allowing the inner sub-amplifier 120to contribute efficiently in the upper amplitude region.

In general terms, this can be obtained by means of letting both L1 andL2 be longer or shorter than one quarter of a wavelength of thefrequency for which the multi-stage amplifier is intended to operate.More specific examples of these lengths will be given in the following.

The electrical length L3 in the second branch is chosen so that itcomplements the effective electrical length L1 or L2. This determines ifChireix characteristics are obtained by the multi stage amplifier 100 ina lower or an upper amplitude region, or in both amplitude regions. Morespecific examples of this length will be given in the following.

Examples of combinations of L1 and L2 which fulfil the conditions givenabove, and which may thus be used in embodiments of the invention arethe following: [L1, L2]=[0.35, 0.35] or [0.15, 0.15] or [0.35, 0.15] or[0.15, 0.35] or [0.2, 0.2], expressed as wavelengths, λ, of thefrequency for which the multi-stage amplifier is intended to operate.

Concerning L3, a suitable length for L3 is that the sum of theelectrical lengths L3 and L1 is one half wavelength, λ, of the frequencyfor which the multi-stage amplifier is intended to operate.

In one embodiment of the multi-stage amplifier 100, in order for theelectrical lengths to fulfil the conditions given above, the electricallengths L1, L2 and L3 are chosen as follows, with A designating thewavelength of the intended operational frequency of the multi-stageamplifier 100:

$\begin{matrix}\left. \begin{matrix}{{L\; 1} = {0.35\mspace{14mu} \lambda}} \\{{L\; 2} = {0.35\mspace{14mu} \lambda}} \\{{L\; 3} = {0.18\mspace{14mu} \lambda}}\end{matrix} \right\} & (1)\end{matrix}$

With such a choice of electrical lengths, Chireix action is obtainedbetween the first 110 and second 120 sub-amplifiers and the thirdsub-amplifier 130.

FIG. 2 shows the RF currents in the three sub-amplifiers as a functionof the amplitude which is obtained by the electrical lengths in theexample given in (1) above, and FIG. 3 shows the efficiency of theentire multi stage amplifier 100 as a function of amplitude with theelectrical length of (1) above.

As can be seen in FIG. 2, the first sub-amplifier 110 draws considerablyless current than the other two sub-amplifiers 120, 130 at higheramplitudes.

As an alternative to the electrical lengths described above in (1), itis also possible to design the multi stage amplifier 100 with electricallengths L1, L2 and L3 which are the complementary lengths to thosestated in (1) above, i.e. electrical lengths L1, L2 and L3 which each is0.5 λ minus the lengths stated above in (1). This will yield the samecurrent and efficiency as those shown in FIG. 2, but the phase movementsof the sub-amplifiers will be reversed.

With renewed reference to FIG. 2, it can be seen that the firstsub-amplifier 110 is the only one which is active at the lower outputamplitudes. This is the same irrespective of the electrical lengthschosen, i.e. either those given above or their complementary lengths.

In a further embodiment, the electrical lengths are instead as follows:

$\begin{matrix}\left. \begin{matrix}{{L\; 1} = {0.35\mspace{14mu} \lambda}} \\{{L\; 2} = {0.15\mspace{14mu} \lambda}} \\{{L\; 3} = {0.29\mspace{14mu} \lambda}}\end{matrix} \right\} & (2)\end{matrix}$

FIG. 4 shows the current vs. amplitude behaviour of the sub-amplifiersin this case, i.e. with the electrical lengths of (2) above; FIG. 5shows the efficiency vs. amplitude of the entire multi-stage amplifierif the electrical lengths are chose as described in that case, i.e. (2)above. As can be seen in FIG. 4, in this case, the third sub-amplifier130 is the one that is active in the lower amplitude regions, and asseen in FIG. 5, a high degree of efficiency is obtained in thisembodiment as well.

Useful combinations of L1 and L2 have also been found to be as follows:L1 and L2 are chosen so that one of them is chosen from one of thefollowing ranges, and the other is chosen from the other of thefollowing ranges: [0.02-0.23], [0.27-0.48], with the lengths in theranges being expressed as wavelengths, λ, of the frequency at which themulti-stage amplifier is intended to operate. Thus, if L1 is chosen fromthe range [0.02-0.23], then L2 should be chosen from the range[0.27-0.48], and vice versa.

In a further embodiment of the multi-stage amplifier of the invention,shown as 200 in FIG. 6, the multi-stage amplifier comprises a fourthsub-amplifier 140, which has an input port 141 and an output port 142.The output port 142 of the fourth sub-amplifier 140 is connected to theoutput port 132 of the third sub-amplifier 130 by means of a connectionwhich has an electrical length L4.

In FIG. 6, which shows the embodiment 200, components which were alsoshown in FIG. 1 have retained their reference numbers.

It will be seen that the first 110 and second 120 sub-amplifiers form afirst “branch” of the multi-stage amplifier 200 which is essentiallyduplicated by a similar second “branch” formed by the third 130 andfourth 140 sub-amplifiers.

Before the characteristics obtained by means of the multi stageamplifier 200 shown in FIG. 6 are described, it will be pointed out thata multi stage amplifier of the present invention can comprise a numberof sub-amplifiers which is more or less arbitrary; in the case of an oddnumber of sub-amplifiers, the principles used for the multi-stageamplifier shown in FIG. 1 should be adhered to, i.e. a number of“branches” like the branch formed by the first 110 and second 120sub-amplifier and an “outer” sub-amplifier connected as the thirdsub-amplifier 130 in FIG. 1.

In the case of an even number of sub-amplifiers, the principlesdescribed in connection to the multi-stage amplifier 200 of FIG. 6should be adhered to, i.e. that of a number of branches which eachconsist of two sub-amplifiers connected as the two branches in FIG. 6.

Returning now to the characteristics obtained by means of the“two-branch” multi-stage amplifier 200 of FIG. 6, the current vs.amplitude characteristics of the first pair of sub-amplifiers 110,140and the second pair of sub-amplifiers 120,130 are shown in FIG. 7. Wesee that sub-amplifiers 110, 140 are the ones that are active at loweramplitudes, and that this pair of sub-amplifiers draws much less currentthan the other pair at higher amplitudes.

FIG. 8 shows the efficiency vs. amplitude characteristic of themulti-stage amplifier 200 of FIG. 6: as seen here, this embodiment ofthe invention also exhibits a high degree of efficiency.

Concerning the electrical lengths of the connections of the multi stageamplifier 200 of FIG. 6, the following have been found to be usefulcombinations: L4 and L1 are chosen from among the following ranges:[0.02-0.23], [0.27-0.48], in which ranges the lengths are expressed aswavelengths, A, of the frequency at which the multi-stage amplifier 200is intended to operate. Thus if L4 is chosen in the range [0.02-0.23],then L1 should be chosen in the range [0.27-0.48], and vice versa.

Similarly, the following have been found to be useful combinations of L3and L2: L3 and L2 are chosen from among the following ranges:[0.02-0.23], [0.27-0.48], in which ranges the lengths are expressed aswavelengths, λ, of the frequency at which the multi-stage amplifier 200is intended to operate. Thus if L3 is chosen in the range [0.02-0.23],then L2 should be chosen in the range [0.27-0.48], and vice versa.

Also, a useful combination of L1 and L4 is to let the sum of L1 and L4be one half of the operational wavelength, λ, of the multi-stageamplifier.

Regarding the sub-amplifiers which are comprised in the differentembodiments of the invention, their exact design can be varied withinthe scope of the invention, as will be realized by those skilled in theart, but a possible realization of a sub-amplifier is shown in FIG. 9.As can be seen, this sub-amplifier comprises a FET-transistor, to whichan LC-circuit (L1, C1 in parallel) is coupled at the drain of thetransistor, and the source of the FET transistor is coupled to ground.The gate of the FET transistor is used as input port Vin for thesub-amplifier, and the drain is used as its output port Vout, via acapacitance C2.

FIG. 9 shows the sub-amplifier of FIG. 1, but with the circuit shown inFIG. 8 as the sub-amplifiers, for the sake of clarity.

As an obvious alternative, the sub-amplifiers 110, 120, 130, 140 can bedesigned using bipolar transistors instead of the FET-transistors whichwere shown in FIGS. 9 and 10.

The invention is not limited to the examples of embodiments describedabove and shown in the drawings, but may be freely varied within thescope of the appended claims.

1. A multi-stage amplifier comprising; a first, a second and a third sub-amplifier, each with respective input and output ports; and the multi stage amplifier also comprising a common output port; wherein the output port of the second sub-amplifier is connected via a first connection having a first electrical length L1 to the output port of the first sub-amplifier and is connected via another a second connection having a second electrical length L2 to the common output port, and the output port of the third sub-amplifier is connected via a third connection having a third electrical length L3 to the common output port; and wherein the electrical lengths L1, L2 of the connections from the second sub-amplifier's output port both to the first amplifier's output port and to the common output port are longer or shorter than one quarter of a wavelength (λ) of the frequency for which the multi-stage amplifier is intended to operate.
 2. The multi stage amplifier of claim 1, in which the electrical lengths L1, L2 of the connections from the second sub-amplifier's output port to the first amplifier's output port and to the common output port are one of the combinations comprising: and, expressed as wavelengths (λ) of the frequency for which the multi-stage amplifier is intended to operate.
 3. The multi-stage amplifier of claim 1, in which a sum of the electrical lengths L3, L1 of the connection from the third sub-amplifier's output port to the common output port and the connection from the output port of the first amplifier to the second amplifier's output port is one half of the operational wavelength (λ) of the frequency for which the multi-stage amplifier is intended to operate.
 4. The multi stage amplifier of claim 1, in which the electrical lengths L1, L2 of the connections from the second sub-amplifier's output port to the first amplifier's output port and to the common output port are such that one of the electrical lengths is one of ranges and, and the other electrical length is the other one of the ranges and, with the lengths expressed as wavelengths (λ) of the frequency at which the multi-stage amplifier is intended to operate
 5. The multi-stage amplifier of claim 1, in which an electrical length L3 of the connection from the output port of the third sub-amplifier to the common output port is 0.18 λ, 0.29 λor 0.32 λ, where λ is the wavelength at which the multi-stage amplifier is intended to operate.
 6. The multi-stage amplifier of claim 1, further comprising a fourth sub-amplifier with an input port and an output port which is connected via a connection having an electrical length L4 to the output port of the third sub-amplifier.
 7. The multi stage amplifier of claim 6, in which the electrical lengths L4, L1 of the connection from the fourth sub-amplifier's output port to the output port of the third sub-amplifier and the connection from the output port of the first sub-amplifier to the output port of the second sub amplifier are one of the ranges comprising and, the ranges expressed as wavelengths (λ) of the frequency at which the multi-stage amplifier is intended to operate.
 8. The multi stage amplifier of claim 6, in which the electrical lengths L3, L2 of the connection from the third sub-amplifier's output port to the common output port and the connection from the output port of the second sub-amplifier to the common output port of the multi-stage amplifier are one of the ranges comprising, in which ranges the lengths are the ranges expressed as wavelengths (λ) of the frequency at which the multi-stage amplifier is intended to operate.
 9. The multi stage amplifier of claim 6, in which a sum of the electrical lengths L4, L1 of the connection from the fourth sub-amplifier's output port to the output port of the third sub-amplifier and the connection from the output port of the first sub-amplifier to the output port of the second sub amplifier is one half of the wavelength (λ) of the frequency for which the multi-stage amplifier is intended to operate.
 10. The multi stage amplifier of claim 6, in which a sum of the electrical lengths L3, L2 of the connection from the third sub-amplifier's output port to the common output port and the connection from the output port of the second sub-amplifier to the common output port of the multi-stage amplifier is one half of the wavelength (λ) of the frequency for which the multi-stage amplifier is intended to operate.
 11. The multi-stage amplifier of any of claim 1, in which at least one of the sub-amplifiers comprises a FET transistor.
 12. The multi-stage amplifier of any of claim 1, in which at least one of the sub-amplifiers comprises a bipolar transistor. 